Over the past few years, wide use has been made of ionomer resins in golf ball cover materials, also referred to hereinafter as “cover stock.”. Ionomer resins are ionic copolymers composed of an olefin such as ethylene in combination with an unsaturated carboxylic acid such as acrylic acid, methacrylic acid or maleic acid, wherein the acidic groups are partially neutralized with metal ions such as sodium, lithium, zinc or magnesium ions. They have excellent characteristics such as durability, rebound and scuff resistance, making them highly suitable as the base resin in golf ball cover stock.
Ionomer resins account for most of the cover stock resin in current use and enable the production of golf balls having the above properties. However, golfers are always on the lookout for golf balls having a high rebound and excellent flight characteristics.
Related improvements taught by the prior art (see U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760, and International Application WO 98/46671) include cover stock in which a large amount of metallic soap is added to the ionomer resin to improve the cost and rebound characteristics of the ionomer cover stock. Such modifications have indeed resulted in better rebound than earlier golf balls with ionomer covers.
However, because a large amount of metallic soap is added to the ionomer resin in this prior-art cover stock, the fatty acids that form due to decomposition of the metallic soap vaporize during injection molding, generating a large amount of gas. The formation of a large amount of gas during injection molding causes molding defects. In addition, gas constituents settle on the surface of the molded article and greatly lower the paintability of the molded article. Moreover, although such cover stock in which a large amount of metallic soap has been added to the ionomer resin does exhibit a rebound which is about the same as or better than that of ionomer having the same degree of hardness, the improvement in rebound is not all that large. Indeed, depending on the type of metallic soap used, the moldability and rebound may in fact be severely compromised and fall far short of practical levels.